Thursday, September 23, 2010

'Tis a Mighty Swale

One of the major assets of our property is a storm water culvert which brings storm water runoff from a number of roads nearby. Water begins to flow through the culvert whenever we have rainfall of more than 8mm. After 25 years of water pouring through, a large gully has been washed away, and this is one of the places in which gorse has found a niche.

This satellite pic shows the course the gully runs and the growth of gorse around it. The main swale bisects the water course and now directs water across the property on contour.
The first priority was to more effectively use the water currently running straight off the property and to slow it down to help with the erosion problem. This led to a design for the mainframe swale, running on contour at the highest point that traversed the longest line across the property. This was partly determined by looking at contour maps, to help decide where we would begin surveying. Online contour maps promised a level line that ran from one far corner to another, beautifully traversing boundary fences and sheds. However, there's nothing like getting outside with a laser level or A-frame... and while pegging it out it began to look somewhat different. South of the gully the swale would run just where we wanted, but on the north side, it ran into the boundary fence. We opted to build the south side swale and come back to the northern paddock at a later date.

Digging the swale at the end of summer, the ground was too hard for the bucket on the little tractor. First the pegged line was ripped a number of times and the loose earth was scooped and piled on the downhill side of the swale.

Scoop and dump ... scoop and dump
Levels were checked by hand by moving along the swale with metal tubing and a spirit level. Constructing swales seems to have a magical meteorological effect... I'm not the first to be rewarded by rain the day a swale is dug. Water is the ultimate spirit level and indicated the need to dig a bit deeper at the bottom end.

The top side of the swale was planted with a variety of acacias - blackwood, black wattle, lightwood and wirilda in the mix. The last thing to do was to make a bridge to the lower paddock.

After years of drought, rainfall this autumn and winter has been constant. The swale has been full since April, resulting in waterlogging in the paddock below. When it came time to plant bare-rooted fruit trees, we decided to mound them up to prevent them drowning. As it turns out, the bottom end of the swale is somewhat higher, which means the ground below is less damp, and a good place to plant cherries, peaches and apricots, which are more susceptible to waterlogging. When things dry out again, the plan is to deepen and widen the swale. Having thought in terms of water scarcity for so many years, I didn't account for how much water would be coming through that culvert. 80mm fell in 24 hours in August, blowing out the walls of small ponds we had constructed to slow down the remaining water that travels through the gully.

This represents a major challenge. We're grappling with how to deal with the force of the water flowing through that small channel in big rain events. There's also a concern about contaminants coming onto the property that have washed off the roads, particularly for the two small dams scheduled for future works. For the time being, the big rains are cleansing, washing up an assortment of rubbish and old bottles that have been thrown in that gully for countless years.


michael chalk said...

Your swales are amazing Sam. Thanks for the pictures and the story.

The satellite photo is interesting too.. i wonder when you'll get a satellite photo of the updated land? That will be good to compare!

i hadn't thought about the contaminants in water from the highway.. is there any way to combat that? Could make it difficult if you ever want to get organic certification.

You are a permaculture genius,

pitchfork projects said...

Ta, Michael! The contaminants are an interesting one, not sure about how big an issue they are, but I think we'll get soil tests done and see what turns up. If we end up digging dams, we'll probably put in some settling ponds to catch what we can of the first flush and then look at what plants are best for to taking up contaminants. Do you know of Paul Stamets, mushroom genius? He's done a lot of work on cleaning toxic waste using fungi.